From the prior art, steer-by-wire systems are known, which are also used in ship control technology. In such systems the steering commands given by a steering unit are detected by a sensor and transmitted by a control unit to a steering actuator which carries out the steering command. Advantageously, there is no mechanical connection between the helm and the rudder, or in the case of a motor vehicle, between the steering-wheel and the steered wheels.
In ship control technology the steering units connected to the rudder are usually hydraulically actuated, which results disadvantageously in poor dynamics and high maintenance costs. In addition steer-by-wire ship's control systems are known, in which the steering actuator is an electro-mechanical actuator in the form of a ball-screw spindle by means of which rotational movement of the one electric motor is converted by the ball-screw spindle into a translational movement for actuating the rudder.
This use of an electric motor driven ball-screw spindle as the steering actuator has the drawback that the adjustment speed and dynamics of the actuator are low; furthermore, ball-screw spindles are not self-locking, especially when the forces acting are large.
For example, a steer-by-wire ship's control system is known from U.S. Pat. No. 6,431,928 B1. In this known system an electric motor is provided for rotating the entire propeller-drive unit by means of a mechanical energy transfer chain, the electric motor being controlled by a control unit connected on the one hand to the steering device to obtain steering command information, and on the other hand to a sensor which detects the steering setting information.
From EP 1770008 A2 a steer-by-wire ship's control system is known, which comprises at least two steering units. In this case the rudder is actuated by means of an actuator which can be operated hydraulically with reference to the steering signals generated by whichever steering unit demands the more rapid movement of the rudder. In the known system the steering units each have a rudder, each of which is connected to a control device which, in turn, is connected to the control network.